Ask Dr. Nick: Can Rotomolding Materials Meet Class A Fire Rating Standards?

Question: We are working on a rotomolding project and need to use a Class A fire-rated material or material that has passed ASTM E84 testing.  I have been speaking with rotomolders who typically use materials with a UL rating rather than ASTM E84. Do you have any information on Class A rotomolding materials or any insight on rotomolding materials that have passed ASTM E84?

Dr. Nick: Fire rating using tests like ASTM E84 are common for construction materials, usually in the form of flat sheets. The method is used to assess the surface burning characteristics of a material used for interior wall and ceiling finishes.  A sheet of material is ignited at one end and the surface burning characteristics are measured by Flame Spread Index (SFI) and Smoke Developed Index (SDI).

With this in mind, it would appear to have limited relevance to testing the fire retardancy of three-dimensional hollow objects, such as those made by rotomolding.  However, I do appreciate that clients with a construction background would be likely to reach for E84 as their “go-to” test for flammability.

As you’ve already found, the flammability of rotomolding materials is more likely to be assessed by a test like UL94, which uses solid coupons of material held in a variety of positions.  This is not to comment on the suitability or otherwise of any test (there are many types of flammability tests), just to explain the lack of readily available data.  I doubt that any manufacturer of rotomolding materials is likely to have data to hand that easily relates to E84.

Having made these caveats, I actually do have some past experience attempting to test typical rotomolding materials to standards somewhat similar to E84.  In the past, I was asked to arrange the testing of some rotomolding materials to British Standard 476 (which has now been discontinued in favor of the similar European Standard ISO 15025).  The methodology of both tests is similar to E84, although there are significant differences in detail.

For the tests, we produced some large three-dimensional products that had flat sides and cut long rectangular test panels from them.  These were then mounted in the test rig and subjected to a specified dose of radiant heat.  One end of the panel was ignited to measure the rate of flame spread.

The majority of plastic rotomolding materials, including those that are produced with flame-retardant additives, are thermoplastics.  This means that they soften and melt as their temperature is raised.  In a flame spread test, the panel is likely to soften and collapse under its own weight before the flame spread can be fully assessed.

I believe that, in the ASTM E84 test, there are three options for mounting specimens: Unadhered, Adhered to GRC (glass-reinforced concrete) and Adhered to Gypsum.  If, with the latter two options, this meant that the material under test could be physically supported by a non-plastic substrate, it might be more feasible to conduct a valid test.

As far as I can ascertain, Class A is the most demanding of the three pass categories within E84.

To summarize:

  1. I believe that it is highly unlikely that rotomolders or their materials suppliers will have flammability data readily available related to ASTM E84.
  2. ASTM E84 is a test designed for flat sheets of construction materials, not three-dimensional plastic objects.
  3. If your clients will not accept a fire test that is more common for three-dimensional objects (ie, UL94), specific testing would need to be commissioned.
  4. It might be possible to test plastic rotomolding materials by E84, if the plastic samples could be supported physically; in principle, this appears to be allowable under some versions of E84.
  5. Given the significant differences between the tests, it is difficult to estimate whether any of the available flame retardant rotomolding grades could give a pass at Class A.

Dr. Nick Henwood, Technical Director of the Association of Rotational Molders, is a 30-year expert in materials and process control. He operates Rotomotive Limited as a consultant, researcher, and educator in the UK and was inducted into the Rotational Molding Hall of Fame in 2022.

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